One method of constructing a hyperspectral imaging instrument is to incorporate a tunable filter in front of a generic two-dimensional detector array such as an infrared focal plane array (FPA), or a detector array responsive to visible or ultraviolet light such as a CCD detector. In FIG. 1 a liquid crystal tunable filter (LCTF) is used which is especially well suited for this purpose, since it allows very narrow wavelength selection (bandpass) over a wide and continuous range, in the visible or near-infrared (NIR) region of the light spectrum. However, this concept is equally applicable for other tunable filters such as AOTFs or interferometers typically used for hyperspectral imaging instrumentation.
In this case, the system is used to measure the NIR light absorbed by a sample at one or more wavelengths. To make such a measurement, it is generally necessary to compare (ratio) the sample response (IS) to the background response (IB) of a non-absorbing reference material (“bright” reference). This removes the contributions arising from the optical properties of the imaging system (illumination source, magnification optics, LCTF, and the FPA), usually referred to as the instrument function. Therefore the sample absorbance (A) is measured as:AS=−Log(IS/IB)  [equation 1]
It has been shown that the accuracy of the absorbance measurement for the sample can be further improved through the subtraction of the stray light signal (ID) which may contribute significant noise to the measurement. The stray light signal is typically measured from a “dark” reference such as a mirror placed significantly out of the focus of the instrument. Incorporating the stray light correction into the measurement, equation 1 becomes:AS=−Log((IS−ID)/(IB−ID))  [equation 2]
The prior art process for collecting data from a sample using the system has been as follows: a sample is placed at the focus of the instrument. The LCTF is tuned to the first desired wavelength and the FPA response is recorded. The LCTF is then tuned to the next desired wavelength and the next FPA response is recorded. This process is repeated for each of the desired wavelengths, and, when completed, the sample data set is stored. Next the sample is removed and replaced with the background reference. The entire process is repeated so that the FPA response at the same wavelength set is collected and stored for the reference. If the stray light correction is to be performed, the background reference is replaced with the “dark” reference, and a third data set is collected in the same stepwise fashion. The order of collection of the three data sets—sample, background, and stray light—is unimportant, each must be completed before the mathematical corrections (either equation 1 or 2) are performed yielding the true absorption response of the sample.